Richard L. Benner

Results of the Gas-Phase Sulfur Intercomparison Experiment (GASIE): Overview of experimental setup, results and general conclusions

Seven techniques for the field measurement of trace atmospheric SO2 were compared simultaneously over 1 month in 1994 using samples produced in situ by dynamic dilution. Samples included SO2 in dry air, in humid air, and in air with potentially interfering gases added. In addition, 2 days of comparison using diluted ambient air were conducted. Six of the seven techniques compared well, with good linear response and no serious interferences but with a range of calibration differences of about 50%.

Modeling of dimethyl sulfide ocean mixing, biological production, and sea-to-air flux for high latitudes

To explore the extent to which ocean mixed-layer dynamics influences dimethyl sulfide (DMS) sea-to-air flux at high latitudes, a model of DMS ocean mixing, biological production, and sea-to-air flux was developed. This biophysical one-dimensional model is driven by meteorology. The model simulates DMS seawater concentrations and vertical distributions, and DMS sea-to-air flux for Prince William Sound and the Gulf of Alaska, from early March through December. Sensitivity analyses revealed that DMS sea-to-air flux is most affected by the rates of flagellate production, Zooplankton grazing, photooxidation, and microbial consumption of DMS. Model results show that under conditions of substantial vertical mixing, such as high wind stress or convective mixing, DMS sea-to-air flux increases significantly. At high latitudes these events may coincide with wind-driven mixing or the overturning of surface seawater due to decreasing sea surface temperatures in the autumn. Parameterizations used to estimate emissions of such a highly variable gas as dimethyl sulfide need to include ocean mixed-layer dynamics. The current model is limited by the small number of DMS loss and production rate measurements available. The measurements that do exist have large ranges and come almost exclusively from low and midlatitude regions, mostly during the summer months under calm conditions. Field measurements are needed from high-latitude systems to refine this model, making it an effective tool for designing field campaigns, improving the accuracy of DMS sea-to-air flux estimations, and assessing the contribution of northern oceans to the atmospheric sulfur budget.

Cloud properties inferred from bimodal aerosol number distributions

Nonprecipitating clouds leave a distinctive fingerprint on the aerosol particles that cycle through them by segregating aerosol particles into two populations, those which are incorporated into cloud droplets and those which are not. This leads to a bimodal or double-peaked character in the aerosol number distribution. If some reasonable assumptions are made, cloud microphysical properties can be inferred from the bimodal aerosol distributions. We have collected over 1700 bimodal distributions from five stations in North America and inferred cloud droplet concentration and maximum supersaturation in the clouds which processed the aerosol particles. Average cloud droplet concentrations are 100–200 cm−3 at “background” stations, while at the polluted site, cloud droplet concentrations were as high as 3000 cm−3. Inferred values of maximum supersaturation ranged from 0.3% at clean sites to 0.1% at the polluted site. Cloud droplet concentration and maximum supersaturation were usually inversely correlated. Cloud droplet concentration and geometric mean diameter of the cloud-processed mode in the aerosol number distribution were also inversely correlated. These two relationships can be understood by comparison with a simple model of cloud activation.

Evidence for sulfuric acid coated particles in the Arctic air mass

In this paper, we discuss results from an experiment conducted on natural aerosol in a clean Arctic environment during the spring. The aerosol behaved as though coated with a volatile film, possibly of sulfuric acid. The thermography of the aerosol observed is consistent with a small fraction of the sulfate being neutralized by ammonium as evidenced by an ammonium to sulfate ratio of 0.14. The volatility behavior of the aerosol is also consistent with an increase in the ammonium to sulfate ratio to 0.39 over one day. The change in the ammonium to sulfate ratio and aerosol volume is consistent with 10 pptv of ammonia in the atmosphere being lost to the aerosol during the period. These values are consistent with others observed in the Arctic winter.

Sulfur gas fluxes and horizontal inhomogeneities in the marine boundary layer

Real-time total gaseous sulfur concentrations were measured (sampling frequency 1 Hz) from the R/V Oceanus as part of the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) field campaign in the Azores during June of 1992. The measurements were used to estimate sulfur gas sea-to-air flux, determine the size scale of sulfur gas inhomogeneities in the marine boundary layer, and the timescale of sampling necessary to characterize a meaningful air mass. Using the time scale of sampling and wind speed measurements, the size scale of sampling can also be determined. Sea-to-air sulfur gas flux estimates were obtained using a variance method and the inertial-dissipation method. Values from five 1-hour measurement periods on two different days ranged from 21 to 28 μmol/m 2 d and 11 to 17 μmol/m 2 d, respectively. These values are above the range of 1 to 13 μmol/m 2 d reported by Blomquist et al. (this issue) during ASTEX/MAGE, based on the dimethyl sulfide (DMS) concentration in surface seawater, using the stagnant boundary layer model of air-sea exchange. A power spectrum of the sulfur gas time series shows the contribution of each turbulent eddy size to the total signal variance. In a representation of the power spectrum, the area under any portion of the curve is proportional to variance. From these power spectra we have obtained the fraction of the total variance in ground level sulfur concentration fluctuations as a function of time. The data indicate that air samples should be integrated for 1000 s for ground-based measurements.

Development and evaluation of the diffusion denuder‐sulfur chemiluminescence detector for atmospheric SO2 measurements

The University of Alaska Fairbanks participated in the Gas-Phase Sulfur Intercomparison Experiment (GASIE) to evaluate the sulfur chemiluminescence detector (SCD). The SCD is a relatively new sulfur selective detection technology (Benner and Stedman, 1989, 1990, 1994; Benner, 1991; Stedman and Benner, 1995). The system used during GASIE is referred to as the diffusion denuder/sulfur chemiluminescence detector (DD/SCD). We chose to use a diffusion denuder to separate SO2 from other sulfur species rather than a gas chromatograph because one of our long-term goals is to develop the DD/SCD into a unique system capable of resolving low pptv levels of SO2 on timescales of 1 min or less. The principles of detection of the DD/SCD and how it was operated during GASIE are presented. Results from the GASIE program are presented in detail by Stecher et al. (this issue). In this paper, we briefly describe what was learned about the diffusion DD/SCD technique from GASIE. To address the problems revealed, the DD/SCD has been completely redesigned and rigorously tested in our laboratory. The modifications made, how these modification eliminate many of the problems found during GASIE, and results form subsequent laboratory testing are discussed.

Gas-Phase sulfur intercomparison Experiment 2: Analysis and conclusions

A diffusion denuder, total sulfur chemiluminescence detector instrument for the measurement of SO2 was tested as the second part of the Gas-Phase Sulfur Intercomparison Experiment (GASIE 2). The SO2 at mixing ratios between 27 and 182 parts per trillion by volume (pptv) was provided by a dynamic dilution apparatus. The data were kept blind from the other party and analyzed by two independent referees. The following was concluded: (1) The independent calibrations of each system are within a few percent. (2) The precision on any one day is better than day-to-day variability. (3) Runs in dry air show a small but significant nonzero intercept in correlation plots. (4) No effects of adding NO2 + O3 or CO2 + CH4 + CO + dimethylsulfide are distinguishable. (5) A small but significant effect due to added H2O is evident in both slope and intercept, but the source could not be discerned. (6) On any one day the systems can distinguish among 0, 20, and 40 pptv, but because of day-to-day variability, they can only distinguish among 0, 30, and 60 pptv on different days.

Subminute measurements of SO2 at low parts per trillion by volume mixing ratios in the atmosphere

The continuous sulfur dioxide detector (CSD) is a sensitive instrument for reliable measurements at high time resolution in the atmosphere. This new instrument is based on a SO2 measurement technique utilizing the sulfur chemiluminescence detector, previously validated in a rigorously blind experiment sponsored by the National Science Foundation. Simplified sample handling, denuder separation technology, and the intrinsic sensitivity and fast response of the detector permit measurement at levels below 100 parts per trillion by volume in tens of seconds with the CSD. The CSD provides a differential measurement where response from ambient SO2 is determined by the difference between air containing SO2 and air scrubbed of SO2, where both air samples contain other detectable sulfur species. Digital signal post processing with phase-locked amplification of the detector signal enhances the precision and temporal resolution of the CSD. Oversampling of the detector signal at 10 Hz permits subsequent data retrieval to be adapted to changing ambient levels by either enhancing signal to noise when sulfur dioxide levels are low or by maximizing temporal resolution of derived data when levels are high. he instrument has advantages over existing instruments based on Chromatographie separation in that the CSD provides accurate and reliable measurements at low parts per trillion by volume sulfur dioxide with high time resolution. The CSD is compact and automated and does not require cryogenic materials, making this instrument suitable for remote field locations. The high temporal resolution, specificity for SO2, and sensitivity of the CSD make it a good candidate for installation on an aircraft. Airborne studies of SO2 with a sensitive, fast time response instrument may offer new insight into the understanding of phenomena such as gas-to-particle conversion, long-range transport of pollutants, and the oxidation of biogenically produced sulfur gases.